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DE.  J.  S.  BOWEEBAJSE:  ox  the  AXATO^klY 
membrane,  between  which  the  first  layer  of  silex  is  secreted,  and  in  this  condition  they 
present  an  internal  cavity  approaching  very  nearly  to  the  size  of  their  external  diameter. 
In  this  state  they  readily  bend  abruptly  in  any  dh'ection  without  breaking,  as  may  be 
seen  in  Plate  XXIII.  fig.  1,  which  represents  a porrecto-temate  spiculum  from  the  tenni- 
nation  of  one  of  the  radial  lines  of  the  skeleton  at  the  surface  of  Tetliea  cranium.  This 
spiculum  has  been  considerably  distorted  by  pressure  on  the  points  of  the  rays  at  its 
apex.  The  deposit  of  the  silex  is  not  continuous  and  homogeneous,  but  is  produced  in 
successive  concentric  layers,  which  it  would  appear  are,  at  least  for  a period,  equally 
secreted  by  the  inner  surface  of  the  outer  membrane  and  the  outer  siuTace  of  the  inner 
one ; for  we  always  find  that  as  the  development  of  the  spiculum  progresses,  the  internal 
ca^dty  gradually  becomes  less,  until  finally  it  exists  only  as  a central  canal  of  very  minute 
diameter  in  comparison  with  that  of  the  spiculum  itself.  These  stages  of  development 
may  often  be  seen  in  the  spicula  of  young  specimens  of  Sjjongilla  fluiiatilis,  especially  . 
in  the  spring,  when  they  are  growing  rapidly.  If  small  fragments  of  the  sponge  be 
slightly  charred  in  the  flame  of  a lamp,  and  then  submitted  to  microscopical  examina- 
tion, the  outer  and  inner  membranes  of  the  spicula  ufill  readily  be  rendered  visible 
(Plate  XXIII.  figs.  2 and  3) ; in  immature  spicula  the  internal  membrane,  is  represented 
by  a dense  black  film  of  charcoal,  as  in  Plate  XXIII.  fig.  3,  while  in  the  matime  ones 
the  small  central  cavity  is  seen  to  be  lined  by  so  thin  a membrane  as  to  afford  by  its 
charring  a slight  brown  tinge  only  to  its  walls  (Plate  XXIII.  fig.  2).  The  concentric 
deposition  of  the  layers  of  silex  or  carbonate  of  lime  in  the  spicula  are  also  readily 
to  be  seen  (Plate  XXIII.  fig.  4)  in  transverse  fractures  of  almost  any  large  spiculum, 
either  siliceous  or  calcareous,  and  they  present  the  same  aspect  as  similar  sections 
of  either  the  prismatic  cells  of  shell  tissue,  or  the  spicula  of  a Gorgonia.  The 
amount  of  silex,  and  the  manner  of  its  deposition  in  the  spicula,  is  not  the  same 
under  all  circumstances.  Where  the  spiculum  is  simply  required  to  give  strength  and 
firmness  to  the  skeleton,  as  in  the  greater  number  of  the  Halichondraceous  sponges, 
the  whole  interior  of  the  spiculum  becomes  rapidly  filled  with  silex ; but  where  strength 
is  required  to  be  combined  with  great  elasticity  and  toughness,  the  mode  of  deposit  is 
especially  adapted  to  the  requirements  of  the  occasion ; the  amount  of  the  silex  depo- 
sited is  small,  and  confined  wholly  to  the  surface,  while  the  interior  appears  to  be 
filled  with  keratode.  These  laws  of  deposit  will  perhaps  be  best  illustrated  by  my 
detailing  a series  of  experiments  I made  by  the  incineration  of  the  spicula  of  various 
sponges  in  the  flame  of  a small  spirit-lamp.  I was  led  to  this  series  of  experiments  by 
frequently  observing  during  the  course  of  my  investigations  the  great  amoimt  of  flexure 
that  many  of  the  large  and  long  spicula  would  sustain  without  fracture,  and  the  perfect 
elasticity  with  which  they  regained  their  original  form  and  position.  Thus,  in  mounting 
the  spicula  of  Tetliea  cranium  in  Canada  balsam,  the  long  and  slender  porrecto-ternatc 
defensive  spicula  projected  from  its  surface  would  frequently  have  the  shaft  bent  into  a 
series  of  sigmoid  curves  or  even  loops ; and  the  thickest  portion  of  the  same  spicula, 
while  in  their  natural  condition,  may  be  bent  domi  to  the  surface  of  the  spbnge,  from 
